Conventionally, multi-stage polishing has been performed generally in the polishing of a silicon wafer. More specifically, the following multi-stage polishing has been performed: a silicon wafer is flattened in the primary polishing, and the surface of the silicon wafer is finished more finely in the secondary polishing and subsequent stages.
In the primary polishing, a high polishing rate is required and flatness of a silicon wafer is demanded. The conventional polishing composition for the primary polishing contains abrasives in order to improve the polishing rate. As the abrasives, nanometer-order colloidal particles or the like are used.
In recent years, as the required accuracy regarding wafer quality increases, the prevention and countermeasure to scratches and LPDs (light point defects) come to be needed in the primary polishing as well.
The mechanical polishing with a polishing composition containing abrasives makes it possible to achieve a high polishing rate, but at the same time, it can be a factor that causes scratches and LPDs.
In considering how to suppress the generation of scratches and reduce LPDs, abrasives have attracted attention, and an abrasive-free polishing composition A for polishing a silicon wafer has been known (Patent Document 1). This polishing composition A is composed of an alkaline aqueous solution that contains a water-soluble silicic acid component and an alkaline component and has a pH of 8.5 to 13.
This polishing composition A is used in a polishing process performed after the polishing of a silicon wafer with a polishing composition B that contains abrasives. In other words, in the two-stage polishing with respect to a silicon wafer, the polishing composition A is used in the latter stage thereof. Therefore, the polishing with a substantially abrasive-free polishing composition has not been realized yet. Further, as a high polishing pressure of about 300 gf/cm2, which is used generally, is required in order to achieve a higher polishing rate, problems arise such as an increase in scratches, and wafer deformation such as edge roll-off. Still further, the polishing under a high polishing pressure involves an increase in the polishing temperature, which causes the reaction of chemical components to increase locally. This leads to problems in the flatness, such as irregularities in the wafer inplane thickness.
In addition, in the multi-stage polishing of a silicon wafer, major challenges in the polishing in latter stages are the elimination and reduction of roughness on the smaller levels, scratches, nanometer-order defects, and LPDs.
Patent Document 1: JP9(1997)-306881A